This invention relates to a process for preparation of dialkylaluminum hydrides. These compounds are known to be suitable co-catalysts in the polymerization of olefins and diolefins and as reducing agents for a variety of organic functional groups, in many cases with a great deal of stereospecificity in the final products.
A number of methods have been proposed or utilized for the preparation of dialkylaluminum hydrides; for example, by reduction of dialkylaluminum halides, by proportionation between a trialkylaluminum and aluminum hydride, by reaction of lithium aluminum hydride with trialkylboron compounds. Also, there have been proposed hydrogenolysis of trialkylaluminum compounds and reaction of trialkylaluminums with metallic aluminum in the presence of hydrogen. These processes all have drawbacks. For instance, reduction of dialkylaluminum halides with an alkali metal hydride is a slow reaction and can be exothermic. Additionally, the presence of solid by-products (alkali metal salts) can complicate product recovery. Aluminum hydride (for use in proportionation) is relatively unstable and may be contaminated with ether. Lithium aluminum hydride and trialkylboron compounds are quite expensive. Hydrogenolysis of trialkylaluminum compounds requires high pressures, for example, 200-300 atmospheres. Reaction of trialkylaluminum compounds with hydrogen in the presence of metallic aluminum requires the use of either activated aluminum or an external catalyst.
A simpler method of preparing some dialkylaluminum hydrides is described in U.S. Pat. No. 3,015,669 and involves subjecting trialkylaluminum compounds to thermal treatment, i.e., pyrolysis. The pyrolysis is conducted at temperatures of from 50.degree. to 200.degree. C, splitting off one molecule of olefin from the trialkylaluminum. Preferably, the trialkylaluminum contains at least one primary branched alkyl radical attached to the aluminum; apparently most preferably all three radicals attached to the aluminum are branched alkyls, and all compounds which were mentioned as being treated had all three radicals identical. It is stated that in many cases it is necessary, or at least advisable, to utilize a catalyst to promote splitting off of the olefin. This is particularly the case with triethylaluminum. Additionally, in the production of diethylaluminum hydride it is expedient not to allow the pyrolysis of the triethylaluminum to proceed to completion but to interrupt the reaction after approximately 50% conversion since the stability of the diethylaluminum hydride decreases as the triethylaluminum starting material is consumed.
Thus, the process of U.S. Pat. No. 3,015,669 is not fully satisfactory for the production of compounds such as diethylaluminum hydride and, by analogy, dimethylaluminum hydride. Additionally, this process is not considered satisfactory for the production of dialkylaluminum hydrides in which the carbon .beta. to the aluminum atom does not have a hydrogen substituent, since the mechanism of hydride formation is known to involve transfer of a .beta.-hydrogen atom to aluminum. Additionally, the general reaction of tri-n-alkylaluminums to produce the aluminum hydride plus an olefin proceeds rather slowly; it has also been found that the equilibrium lies more heavily on the side of the starting material than the products and that olefin elimination is complicated by addition of the carbon-aluminum bond of the trialkylaluminum to the olefin formed.
It is an object of the present invention to provide an improved process for the production of dialkylaluminum hydrides.
It is a further object of the present invention to provide a process for preparation of dialkylaluminum hydrides in which the carbon atom .beta. to the aluminum atom is not substituted with hydrogen.
A further object of the present invention is to provide an improved process for the production of diethylaluminum hydride.
Another object of the present invention is to provide an improved process for the production of di-n-alkylaluminum hydrides.
Yet a further object of the present invention is to provide a process for the production of dialkylaluminum hydrides of relatively high purity and in relatively good yield.
A still further object of the present invention is to provide a process for the production of dialkylaluminum hydrides which does not require the presence of a catalyst.
Other objects and advantages of the invention will be apparent from the description which follows.